The present invention relates to a train radio communication system of a diversity reception scheme and, more particularly, a train radio communication system for always holding the optimum communication quality in a train communication system.
FIG. 7 shows a conventional train radio communication system.
Various communicating schemes for a train radio communication system have been proposed and put into practice. Of these schemes, a space diversity reception scheme is highly evaluated. The basic arrangement of the space diversity reception scheme will be described below with reference to FIG. 7.
Referring to FIG. 7, a large number of land communication equipments 31 are installed at given intervals along a railroad on which a train travels, and a train communication equipment 32 performs communication while sequentially switching the land communication equipments 31 with which the train communication equipment 32 communicates during travel. FIGS. 7 shows a case wherein the train communication equipment 32 is communicating with one of the plurality of land communication equipments 31. The train communication equipment 32 comprises an antenna unit 321 constituted by a plurality of antennas 321a to 321n, a reception unit 322 constituted by a plurality of receivers 322a to 322n, a channel monitor unit 323 constituted by a plurality of channel monitors 323a to 323n, a channel quality comparator 324 for specifying a reception route having the optimum quality by comparing output results from the channel monitors 323a to 323n of the channel monitor unit 323, and a receiver selection switch 325 for selecting one of reception outputs from the receivers 322a to 322n on the basis of the comparison result obtained by the channel quality comparator 324.
The antennas 321a to 321n, the receivers 322a to 322n, and the channel monitors 323a to 323n are arranged in a one-to-one correspondence, and respectively constitute independent reception routes. That is, an RF signal modulated/transmitted by/from the land communication equipment 31 is sent to the receivers 322a to 322n via the antennas 321a to 321n in the train communication equipment 32 to be demodulated and output as reception outputs. The channel monitors 323a to 323n evaluate the qualities of the reception outputs and output the respective qualities as numerical values. Although various quality evaluation methods are available in this case, a method based on received power level is often used for an analog scheme; and a method based on a bit error rate (BER) or the like, for a digital scheme.
The channel quality comparator 324 compares the received power levels or BERs in the respective reception routes, sent from the channel monitors 323a to 323n, with each other to specify one reception route which is evaluated as a reception route having the highest channel quality. The receiver selection switch 325 receives all the reception outputs from the receivers 322a to 322n and has a function of selecting and outputting only one reception output from the receiver belonging to the reception route specified by the channel quality comparator 324.
The antennas 321a to 321n in the antenna unit 321 are arranged at spatially different positions. In general, multiplexed wave propagation occurs in mobile communication including train radio communication, resulting in a deterioration in communication quality. However, with the arrangement of a plurality of antennas installed at spatial different positions, even if the channel quality of a reception route including a given antenna deteriorates owing to multiplexed wave propagation, there is a possibility that good channel quality is maintained in some reception route including another antenna, because different multiplexed wave arrival environments are set in the respective reception routes. Therefore, optimum channel quality can be expected to be maintained by selecting a reception route having a high received power level or a reception route having a low BER with the arrangement described above.
In addition, a diversity reception unit including at least four directional antennas and three receivers is disclosed in Japanese Patent Laid-Open No. 61-72421. In this equipment, while diversity reception is performed by using two receivers, a search for a high-output antenna is made by using the remaining one receiver, thereby realizing better reception.
In such a conventional diversity reception unit, however, a plurality of reception routes, i.e., a plurality of antennas and a plurality of receivers and channel monitors are required. Consequently, the train communication equipment increases in size and cost.